Persistent fine-scale fault structure and rupture development: A new twist in the Parkfield, California, story

• Published in: Earth and planetary science letters Vol. 521; pp. 128 - 138
• Main Authors: Perrin, Clément, Waldhauser, Felix, Choi, Eunseo, Scholz, Christopher H.
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 01-09-2019; Elsevier
• Subjects: Astrophysics; Earth and Planetary Astrophysics; fault structure; long-term deformation; Parkfield earthquakes; rupture initiation and arrest; San Andreas Fault; Sciences of the Universe; San Andreas Fault; fault structure; long-term deformation; rupture initiation and arrest; Parkfield earthquakes
• ISSN: 0012-821X; 1385-013X
• DOI: 10.1016/j.epsl.2019.06.010

We investigate the fine-scale geometry and structure of the San Andreas Fault near Parkfield, CA, and their role in the development of the 1966 and 2004 ∼M6 earthquakes. Long-term surface fault traces indicate that structural heterogeneities associated with secondary reverse and normal fault structures are present at both rupture tips, near Middle Mountain and Gold Hill. Detailed analysis of almost 50 years of high-resolution seismicity reveals a fault plane that has been twisted into a helicoid between Middle Mountain and Gold Hill. Numerical models support our conclusion that this shape is the result of long-term torqueing of a strong stuck patch surrounded by a weak creeping region. The changes in fault friction behavior and related geometric discontinuities act as barriers to rupture propagation of moderate size earthquakes at Parkfield, and as areas of concentrations where rupture initiates. Our study demonstrates also that smooth strike-slip faults with large cumulative offset can form new fault segments at a late stage in their evolution.
•The San Andreas fault plane near Parkfield has been deformed into a helicoid over the long-term.•The torque results from the strength contrast between locked and creeping zones.•Complex structures at surface correspond with the tips of the stuck patch at depth.•Stress concentrations at the tips of the stuck patch control rupture development of M6 events.